Laminate and process for producing a laminate of this type

ABSTRACT

The invention relates to a laminate, comprising a plastic layer which contains a thermoplastic, which plastic layer is joined at least on one side to a substantially parallel metal skin. The invention also relates to a metal-plastic-metal laminate of this type. According to the invention, the core layer comprises, in addition to the thermoplastic, a solvent and a hardener, the thermoplastic being dissolved in the solvent in order to make the thermoplastic processible, during production of the laminate, at a temperature which is lower than the glass transition temperature of the thermoplastic, and the hardener being capable of reaction with the solvent, or in that the core layer comprises polymer particles which are dispersed in the thermoplastic, which polymer particles are formed from a solvent and a hardener which is capable of reaction with the solvent. In this way, the core layer becomes easier to work during the production of the laminate, while the laminate itself, after cold-working, is dimensionally stable at elevated temperature. The invention also relates to a process for producing a laminate of this type.

[0001] The invention relates to a metal-plastic-metal laminate,comprising a core layer containing a thermoplastic, which core layer issituated between two substantially parallel metal skins and is joinedthereto. The invention further relates to a laminate comprising aplastic layer which contains a thermoplastic, which plastic layer isjoined on one side to a substantially parallel metal skin. The inventionfurther relates to a process for producing a laminate of this type,including a metal-plastic-metal laminate.

[0002] Patent Publication EP 0 598 428 describes a metal-plastic-metallaminate. The described laminate comprises two metal skin plates, eachwith a thickness of between 0.08 and 0.3 mm, between which there is acore layer of solid polypropylene (PP) with a thickness in a range from0.5 to 2.0 mm, and a layer of adhesive between the core layer and theskin plates. This known laminate can be used as a laminate for producingbodywork parts.

[0003] A laminate comprising a thermoplastic containing layer that isjoined on one side to a substantially parallel metal skin can be usedfor certain other bodywork parts.

[0004] The processing to form usable products, of the laminatecomprising a layer containing a thermoplastic joined to at least onemetal skin, such as bodywork parts, structural sheets and structuralsections for walls and roofs and other objects, generally comprisescold-working followed by coating of the components. A coating treatmentcarried out on automobile bodies and components therefor made from steelcomprises a cataphoresis treatment, a filler treatment and a topcoattreatment. The cataphoresis treatment usually takes place at 180° C.,while a curing step at a temperature of between 180 and 200° C. usuallyforms part of the topcoat treatment. Mechanical stresses are formed inthe laminate as a result of the cold-working.

[0005] One requirement for a laminate is that a product producedtherefrom, after it has been converted into the desired form, bedimensionally stable during one or more of the subsequent treatments atthe corresponding temperatures. The term dimensionally stable isunderstood as meaning that the laminate, in a test which is described inEP 0 598 428 during which a specimen of the laminate is firstlycold-worked and then, during a subsequent heat treatment at a settemperature, changes in shape by no more than 0.5°.

[0006] One drawback of the known metal-plastic-metal laminate is that itis insufficiently dimensionally stable at a temperature of over 145° C.Another drawback of the known laminate is that an additional adhesionlayer is always required between the core layer and the metal skins.

[0007] It is an object of the invention to provide an improved laminate.It is another object to provide a laminate which is dimensionally stableat a temperature of at least 180° C. It is yet another object to providea laminate which is suitable for cold-working. It is yet another objectof the invention to provide a laminate in which a layer of adhesive isnot necessary. It is yet a further object of the invention to provide alaminate which is suitable for producing structural and bodywork parts,and possibly other products into which sheet metal is processed, such aspackaging items.

[0008] One or more of these objects is achieved with ametal-plastic-metal laminate, comprising a core layer with athermoplastic, which core layer is situated between two substantiallyparallel metal skins and is joined thereto, wherein the core layerfurthermore comprises a solvent and a hardener, the thermoplastic beingdissolved in the solvent, and the hardener being capable of reactionwith the solvent.

[0009] The solution of the thermoplastic and the solvent has a lowerviscosity than the thermoplastic and is therefore easier to process thanthe thermoplastic itself. The result is a laminate in which the corelayer comprising a thermoplastic, at least during production of thelaminate, can be processed at a temperature which is lower than theglass transition temperature of the thermoplastic without solvent.

[0010] There is a broad choice of plastics which are stable at a hightemperature. However, an inherent problem of these stable plastics isthat they are difficult to process at these and lower temperatures. Theprocessibility of the laminate according to the invention at lowertemperatures is improved by the solvent compared to the laminate inwhich there is no solvent. Making the solvent react with the hardener toform a reaction product, according to the invention, changes the solventin such a manner that the plastic is no longer, or at least is less,soluble therein, resulting in a laminate with a higher dimensionalstability than the laminate before the reaction took place.

[0011] The solvent and the hardener preferably each comprise at leasttwo reactable groups. This ensures that the hardener can be polymerizedwith the solvent, forming a reaction product which comprises polymerchains.

[0012] The reaction product which is formed from the solvent and thehardener is preferably not miscible with the plastic. As a result, whenthe solvent is reacted with the hardener, the undissolved thermoplasticis formed, with its own properties, since the solvent is separated outof the thermoplastic.

[0013] The solvent preferably comprises a low molecular weight epoxyresin. Epoxy resins have proven to be very effective solvents for alarge number of thermoplastics. Moreover, epoxy resins are easy topolymerize, with the result that the solvent can easily be removed fromthe plastic. Moreover, epoxy groups exhibit good adhesion to metalsurfaces. The good adhesion of epoxy groups to metal surfaces leads togood adhesion between the core layer and the metal skin without anadditional adhesion layer being required. Dispensing with a separateadhesion layer has a beneficial effect on the cost price of thelaminate, in particular because the production process is simpler.

[0014] The hardener preferably belongs to the group of amines, the groupof amides or the group of anhydrides. Hardeners of this type reactexcellently with epoxy groups. Moreover, hardeners of this type arecommercially available, in a broad selection, at favourable prices. Thetemperature and rate at which these hardeners react with the epoxygroups depend, inter alia, on the type of hardener, so that a suitablelaminate according to the invention is possible for various types ofapplications. For example, with anhydrides epoxy resins usually hardenat a temperature which is typically higher than 180° C., so that it isthus possible to achieve a laminate in which the core layer has not yethardened at lower temperatures and room temperature.

[0015] The laminate which has been described hitherto is anintermediate. As long as the plastic is dissolved in the solvent, thelaminate does not yet have the high dimensional stability which isultimately desired.

[0016] The invention is also embodied by a laminate, comprising a corelayer which contains a thermoplastic, which core layer is situatedbetween two substantially parallel metal skins and is joined thereto, inwhich metal-plastic-metal laminate the core layer comprises polymerparticles which are dispersed in the thermoplastic, which polymerparticles are formed from a solvent and a hardener which is capable ofreaction with the solvent. The result is a metal-plastic-metal laminatein which the skin plates are joined to one another via a network ofuninterrupted paths through the thermoplastic in the core layer, thesolvent having been separated out of the thermoplastic. As a result, theultimately desired dimensional stability is achieved.

[0017] During the cold-working of the metal-plastic-metal laminate,mechanical stresses are formed in the metal-plastic-metal laminate. Thenetwork of uninterrupted paths through the dimensionally stable plasticcreates a rigid bond between the two metal skins. As a result, the corelayer is able to retain the above stresses without allowing relaxation.

[0018] The thermoplastic in the laminate preferably only has anamorphous phase. If a crystalline phase is possible as well as anamorphous phase, there is a risk that nuclei will actually form from thecrystalline phase. The formation of crystalline nuclei isdisadvantageous, since the density of the crystalline phase differs fromthe density of the amorphous phase. The difference in density results inhigh internal stresses in the core layer, which internal stressesreadily cause deformation of the laminate.

[0019] The thermoplastic in the laminate preferably has a glasstransition temperature which lies in a range from 180° C. to 240° C.This ensures that the laminate retains its dimensional stability attemperatures which are usually employed after the cold-working in aproduction line for bodywork parts.

[0020] The thermoplastic more preferably has a glass transitiontemperature which lies in a range from 200° C. to 220° C. In this way,subsequent treatments during which the temperatures rise to between 180°C. and 200° C. can be carried out while maintaining the form of thelaminate as laid down by the initially imposed requirement. A laminatewith a core layer comprising a thermoplastic with a glass transitiontemperature which is higher than the temperatures which occur during thesubsequent treatment is able to withstand the treatment for a longertime while retaining its shape than a laminate in which the plastic hasa lower glass transition temperature. The advantage of a glasstransition temperature which is not too far above the maximumtemperature of a subsequent treatment is that if desired it is alsorelatively easy to carry out other subsequent treatments which involveraising the temperature to above the glass transition temperature.

[0021] In one embodiment of the laminate, the thermoplastic belongs tothe group of poly(phenylene ethers), includingpoly(2,6-dimethyl-1,4-phenylene ether), known as PPE for short. PPEsatisfies the demands set out above. It has only an amorphous phase andhas a glass transition temperature of approximately 220° C. If desired,a similar plastic in which the non-reactive groups on the phenylenediffer from the 2,6-dimethyl structure can be used, in order, forexample, to achieve a different glass transition temperature.

[0022] The core layer of the metal-plastic-metal laminate preferably hasa thickness in a range from 0.5 to 2.5 mm. The result is ametal-plastic-metal laminate in which the metal skins are sufficientlyseparate from one another to produce the required rigidity of thelaminate. Compared to a metal sheet of the same rigidity as thecorresponding metal-plastic-metal laminate, the correspondingmetal-plastic-metal laminate is lighter, since the core layer has arelatively low density. The thickness is more preferably in a range from0.5 to 1.0 mm. The result is a metal-plastic-metal laminate which issufficiently rigid to satisfy the requirements for bodywork parts forcars, for example, in which as little plastic as possible is processed.This reduces the cost price of the metal-plastic-metal laminate.

[0023] A metal skin preferably consists of steel, galvanized steel,stainless steel or an aluminium alloy. These are materials which arealready being used in vehicle bodies, so that further process steps inthe production of vehicle bodies do not have to be adapted. Thesematerials are also employed in the field of packaging production.

[0024] Certain aluminium alloys are particularly suitable for hardeningby thermal means, and certain aluminium alloys are particularly suitablefor work-hardening. On account of the way in which a laminate is usuallyproduced, the aluminium alloy preferably belongs to the group ofaluminium alloys which are particularly suitable for work-hardening. Abetter laminate is produced as a result. Examples of suitable aluminiumalloys are Aluminium Association 5182, 5054, 6063 and 6016. Aluminium ofthis type has the appropriate hardness and properties for the intendedapplications. It is recommended that the laminate comprise prestretchedaluminium, in order to prevent so-called Lüders lines from developingduring the cold-working of the laminate.

[0025] In one embodiment of the metal-plastic-metal laminate, the metalskin has a thickness in a range from 0.08 to 0.3 mm. The result is ametal-plastic-metal laminate which is eminently suitable for producingproducts for a wide range of applications. The thickness of the metal ispreferably between 0.12 and 0.19 mm. The result is a metal-plastic-metallaminate which is eminently suitable for bodywork parts in, for example,the automotive industry.

[0026] In one embodiment of the laminate, the laminate comprises anadhesion layer between the core layer and the metal skin. This improvesthe adhesion between the core layer and a metal skin, in order toprovide a laminate for applications in which better adhesion isdesirable. Moreover, improved adhesion between the core layer and themetal skin contributes to improving the dimensional stability of thelaminate as a whole.

[0027] In an embodiment in which the core layer comprises athermoplastic which belongs to the group of poly(phenylene ethers), theadhesion layer preferably belongs to the group of polyurethanes (PUR).Unlike epoxy resin, for example, PUR adheres well to poly(phenyleneethers).

[0028] The invention is also embodied in a laminate comprising a plasticlayer which contains a thermoplastic, which plastic layer is joined onone side to a substantially parallel metal skin, wherein the plasticlayer furthermore comprises a solvent and a hardener, the thermoplasticbeing dissolved in the solvent, and the hardener being capable ofreaction with the solvent; and the invention is also embodied in alaminate comprising a plastic layer which contains a thermoplastic,which plastic layer is joined on one side to a substantially parallelmetal skin, wherein the plastic layer comprises polymer particles whichare dispersed in the thermoplastic, which polymer particles are formedfrom a solvent and a hardener which is capable of reaction with thesolvent.

[0029] A laminate of a plastic layer joined on one side to a metal skin,can be designed and manufactured such that the mechanical strength isprovided mainly by the plastic, whereas the outside appearance of thelaminated product is metal like. For this purpose a metal skin being ametal foil having a thickness of between 0.020 mm and 0.080 mm issufficient. Such a laminate is, amongst other applications, verysuitable for manufacturing an automobile bumper.

[0030] The above described technical features with respect tocomposition and treatment of the thermoplastic containing core layer ofthe metal-plastic-metal laminate according to above described perferredembodiments of the invention, and the choice of metals of themetal-plastic-metal laminate according to above described perferredembodiments of the invention, are also preferred technical features forthe laminates wherein, instead of the plastic layer being situatedbetween two metal skin layers, the plastic layer is joined to asubstantially parallel metal skin on one side only leaving the otherside substantially free of a skin, or wherein the plastic layer issituated between a metal skin and a non-metallic skin or a bodycomprising a non-metallic material. The non-metallic material may beanother plastic, or wood, or glass.

[0031] In a further aspect, the invention relates to a process forproducing a laminate comprising a thermoplastic containing layer that isjoined at least on one side to a substantially parallel metal skin,including a metal-plastic-metal laminate. According to the invention,the process comprises the steps of:

[0032] i. providing granules of a thermoplastic, a solvent and ahardener, which hardener is capable of reaction with the solvent;

[0033] ii. dissolving the granules in the solvent, and adding thehardener to form a mixture;

[0034] iii. extruding the mixture in order to achieve a sheet form;

[0035] iv. forming a laminate comprising a layer of the extruded mixtureand at least one substantially parallel metal skin.

[0036] As a result of the thermoplastic being dissolved in a solvent,the viscosity at a specific processing temperature becomes lower thanthe viscosity of the thermoplastic itself. As a result, the core layerwith the dissolved thermoplastic can be processed and extruded moresuccessfully at lower temperatures than a core layer comprisingundissolved plastic. It is now possible to select a thermoplastic with ahigh glass transition temperature for production of a laminate, duringwhich certain steps involved in production may nevertheless take placeat a low temperature.

[0037] It is remarked that this process is understood to include aprocess for the production of the metal-plastic-metal laminate accordingto the invention, wherein step iv amounts to forming ametal-plastic-metal laminate comprising a core layer of the extrudedmixture.

[0038] Dimensional stability of the laminate can also be achieved athigher temperature. For this purpose, one embodiment of the processfurthermore comprises the steps of:

[0039] v. heating the laminate in order to make the hardener react withthe solvent; and

[0040] vi. cooling the laminate to a processing temperature.

[0041] This terminates the dissolving function of the solvent, since thelow molecular weight epoxy resin and the hardener together form apolymer. Then, the cooling causes the core layer to adopt a desireddimensional stability. Preferably, the laminate is thus suitable forcold-working while having sufficient dimensional stability to besubjected to a conventional coating treatment after the cold-working,without mechanical stresses which have built up in the laminaterelaxing.

[0042] The heating of the laminate in order to make the hardener reactwith the solvent is preferably carried out at the same time as theformation of the laminate. The result is that a dimensionally stablelaminate is produced using a quicker and easier production process. Thelaminate obtained in this way can still be cold-worked to form aproduct, but the shape of the product remains stable during subsequentheat treatments.

[0043] A further advantage of forming the laminate at the same time ascarrying out the reaction between the solvent and the hardener is thatthe hardener and/or the solvent is/are thus also available for bondingto the surface of a metal skin. As a result, the core layer adheres to ametal skin with the aid of these constituents, so that it is possible todispense with the need to apply a separate adhesion layer between thecore layer and the metal skin.

[0044] In another embodiment of the process, step v is carried outbefore step iv but after step iii. The result is an intermediate productof a hardened, extruded sheet, which can subsequently be processed toform a laminate.

[0045] The reaction between the hardener and the solvent preferablytakes place at a temperature which is higher than the glass transitiontemperature of the thermoplastic. Since the viscosity of the plasticfalls as the temperature rises, mass transfer and diffusion processes inthe plastic are accelerated, so that a thermally and/or chemicallyinduced phase separation between the plastic and the reaction productbecomes possible. As a result, polymer particles are formed, it beingpossible for the polymer particles to be dispersed within the plastic.

[0046] In one specific embodiment, the plastic is selected from thegroup of poly(phenylene ethers), such as PPE, and the solvent isselected from the group of low molecular weight epoxy resins, and thehardener is selected from the group consisting of amines, amides andanhydrides.

[0047] PPE only has a amorphous phase and has a glass transitiontemperature of approximately 220° C.

[0048] Epoxy resins adhere well to metal surfaces. Moreover, it islikely that during the hardening the epoxy resin preferentially binds inthe vicinity of the metal surface. Both aromatic resins (for examplebisphenol-A-diglycidyl ether, BADGE) and aliphatic resins (for examplepolypropylene oxide diglycidyl ether, PPODGE), as well mixtures thereof,can be used. The aromatic epoxy resins are harder than the aliphaticepoxy resins and, in addition, less expensive.

[0049] Amines, amides and anhydrides are widely commercially availableat favourable prices. Hardeners of this type react excellently withepoxy groups. The temperature and rate at which these hardeners reactwith the epoxy groups depend, inter alia, on the type of hardener, sothat it is possible to provide a suitable laminate for various types ofapplications. Both aromatic amines (for example4,4′-methylene-bis(2,6-diethylaniline) M-DEA, or4,4′-methylene-bis(3-chloro-2,6-diethylaniline) M-CDEA), and aliphaticamines (for example propylene oxide diamine PODA or polypropylene oxidediamide), as well as mixtures, can be used. Aromatic compounds areslower on account of steric hindering.

[0050] The molecular weight of the poly(phenylene ether) selected ispreferably between 20 000 and 30 000 atomic mass units (amu). A plasticwith PPE chains of this type of length has the correct viscosity for thecore layer of the laminate. Below 20 000 amu, the phase separation hasbeen found to be too slow for the above applications. Above 30 000 amuthere are in relative terms too few terminal phenolic groups. However,these terminal groups are desirable, since they are able to react withthe epoxy particles, resulting in secure anchoring between the epoxyparticles and the plastic, which is of benefit for the adhesion of thecore layer to the metal skins. Moreover, the viscosity of the mixturebecomes too high at a high molecular weight, thus reducing theapplicability and extrudability at a defined temperature.

[0051] The mixture preferably comprises between 60 and 80% by weight ofpoly(phenylene ethers). At higher percentages than 80%, the melt can nolonger be sufficiently processed, while at lower percentages than 60%,the ultimately desired dimensional stability is not achieved, since anexcessively large fraction of epoxy particles is dispersed in the corelayer, making the plastic network less strong. The percentage ofpoly(phenylene ethers) in the mixture is preferably approximately 70% byweight. The result is a dimensional stability which is sufficient forthe above applications, while the dissolved plastic also remainssufficiently processible and extrudable.

[0052] The extrusion of the mixture comprising PPE and epoxy resin ispreferably carried out at a temperature in a range from 160° C. to 200°C., and more preferably in a range from 170° C. to 180° C. Thistemperature is just slightly lower than the temperature to which thelaminate is subjected in a coating operation.

[0053] The temperature at which the PPE granules are dissolved in theepoxy resin preferably lies in a range from 170° C. to 200° C. Theresult is a homogeneous composition. A particular advantage is achievedif the extrusion is also carried out in a temperature range of thisnature, since the mixing and extrusion can then be carried out in thesame device.

[0054] The laminate with a PPE-containing core layer is preferablyformed at a temperature which is higher than 240° C. This temperature isa good deal higher than the glass transition temperature of PPE, withthe result that the plastic flows sufficiently and can bed itself snuglyagainst the metal skin.

[0055] The invention will now be explained with reference to thedrawing, in which:

[0056]FIG. 1 provides an overview of structural formulae of some of thecompounds mentioned in the description; and

[0057]FIG. 2 diagrammatically shows a cross section through themetal-plastic-metal laminate according to the first aspect of theinvention, before and after the formation of the polymer particles.

[0058] One of the objects of the invention is to provide laminatecomprising a thermoplastic containing layer, which laminate isdimensionally stable at a temperature of at least 180° C.

[0059] Many plastics exhibit a considerable fall in viscosity even attemperatures much lower than the melting point, on account of theirlower glass transition temperature. One possible solution to thisproblem is to add substances which limit the fall in viscosity of theplastic in the temperature range around the glass transitiontemperature. However, this leads to unnecessarily expensive andspecialist handling of plastics.

[0060] A problem with plastics having a sufficiently high dimensionalstability and a sufficiently high glass transition temperature is thatinsufficient extrudability and processibility at a temperature which iscustomary in industry is inherent to a plastic of this type.

[0061] Even if a plastic having the required high dimensional stabilityis selected, and sufficiently high temperatures are used during theproduction of the laminate to process such a plastic, the plastic mayburn already before it can be made sufficiently extrudable as a resultof the temperature being raised to the appropriate range.

[0062] The present invention at least partly resides in that the corelayer comprises, in addition to a thermoplastic, a solvent and ahardener which is capable of reaction therewith. As a result, theprocessing range of the thermoplastic is temporarily shifted towardslower temperatures, since the dissolved plastic has a lower viscositythan the plastic itself. When the ultimately desired high dimensionalstability is to be reached, according to the invention the solvent canbe removed from the plastic with the aid of a polymerization reactionbetween the hardener and the solvent.

[0063] To do this, it is preferable to select a system of plastic andhardener/solvent in which the reaction product formed from the hardenerand solvent are not miscible with the plastic. The result is a phaseseparation which may even involve phase inversion. As a result, thereaction product in the form of polymer particles becomes dispersed inthe plastic.

[0064] Mixtures of thermoplastics of this type, and segregation thereofwith the aid of chemically or thermodynamically induced phaseseparation, are known, for example, from the article “Processing ofthermoplastic polymers using reactive solvents” by H. E. H. Meijer etal., published in 1996, in Volume 8 of “High Performance Polymers”. Theidea of producing a dimensionally stable laminate or metal-plastic-metallaminate from exactly such a mixture is altogether novel.

[0065]FIG. 1 shows an overview of structural formulae of a number ofcompounds which are referred to in the description. These are (a) theplastic PPE, (b) an aromatic epoxy resin BADGE, (c) an aliphatic epoxyresin PPODGE, (d) an aliphatic hardener polypropylene oxide diaminePODA, and (e), (f) aromatic hardeners M-CDEA and M-DEA.

[0066]FIG. 2a diagrammatically depicts a metal-plastic-metal laminateaccording to the first aspect of the invention. It comprises twosubstantially parallel metal skins 1, between which there is a corelayer 2 which is connected to the metal skins. In FIG. 2a, the corelayer comprises a solution of a thermoplastic in a solvent mixed with ahardener which is capable of reaction with the solvent.

[0067] After the core layer has been heated, for example after theformation of the laminate, the hardener reacts with the solvent.Particularly at a temperature which lies above the glass transitiontemperature of the plastic, thermally and/or chemically induced phaseseparation or even phase inversion occurs, resulting in the laminatewhich is diagrammatically depicted in FIG. 2b. The core layer in FIG. 2bcomprises a continuous plastic layer 3, in which the polymer particles 4formed from the solvent and the hardener are dispersed.

[0068] In other embodiments of the invention, only one side of theplastic layer that is shown in FIGS. 2a and 2 b is joined to a metalskin layer. In yet another embodiments of the invention, one of themetal skin layers shown in FIGS. 2a and 2 b is replaced by anon-metallic skin or a body comprising a non-metallic material.

[0069] According to the invention, in particular poly(phenylene ethers),such as PPE, are preferred as the thermoplastic base for the core layer.On the one hand, it is desired for the molecular weight of thepoly(phenylene ethers) to be as high as possible, since a high molecularweight promotes the phase separation of the hardener/epoxy particlesformed. However, a certain fraction of terminal phenolic groups is alsodesirable, since they can bind themselves to the epoxy particles. Thistypically occurs at a temperature of approximately 200° C. The adhesionof the core layer to the metal skins is improved as a result. Moreover,the viscosity of poly(phenylene ethers) of lower molecular weight islower, so that less solvent is required to achieve the same viscosityand it is possible for a higher fraction of the core layer to be formedfrom the poly(phenylene ethers) with the high dimensional stability. Ithas been found that a molecular weight of between 20 000 and 30 000 amugives good results for the abovementioned applications.

[0070] In addition to poly(phenylene ethers), various types ofpolyamides also exhibit a sufficiently high dimensional stability at atemperature of between 180° C. and 200° C. in order, if desired, to besuitable for consideration as the base material. However, for bodyworkapplications, it is preferable to use PPE types, since they have ahigher moisture resistance than the said polyamides.

[0071] Many plastics have both an amorphous phase and a crystallinephase. When the amorphous phase is transformed to a crystalline phase,the result is local density differences in the plastic, leading tointernal stresses. It is possible to counteract the formation ofcrystalline nuclei and the growth of this phase as far as possible withthe aid of additives, but according to the invention it is preferable toselect a plastic which does not have a crystalline phase.

[0072] In particular for the production of a metal skin comprisinglaminate, the use of an epoxy resin as solvent has a particularadvantage, since it has been found that the epoxy particles bindthemselves preferentially in the vicinity of a metal surface, duringwhich process they also adhere to the metal surface. Therefore, evenafter they have been removed from the thermoplastic as solvent, theyform an important constituent of the core layer, since they promote theadhesion of the core layer and the metal. Consequently, it isunnecessary to arrange an additional adhesion layer between the corelayer and a metal skin, leading to reduced production costs.

[0073] The amount of hardener which is preferably present in the mixtureis usually adapted in approximately stoichiometric proportions to theamount of reactable solvent. If desired, it is possible to deviate fromthese proportions. It has been found that, particularly in the case ofaromatic compounds, a proportion of the hardener is absorbed in theplastic and is therefore not available for bonding with the solvent. Insuch cases, it is recommended to adjust the mixing ratio accordingly.Typical glass transition temperatures of polymer particles formed fromepoxy resin and amines are around 180° C., depending on the degree ofcrosslinking.

[0074] To produce the laminate according to an embodiment of theinvention, granules of the desired thermoplastic particles aredissolved. In practice, this can be carried out in a twin-screwextruder, for example. In the case of PPE and epoxy resin, this mostsuitably takes place at a temperature of between 170° C. and 200° C. Inthe case of difunctional epoxy resin, an amount of between 10 and 20% byweight will create a sufficiently low viscosity. When a homogeneouscomposition has been reached, the desired, approximately stoichiometricamount of hardener can be added to the solution, preferably just beforethe extrusion takes place, and then a sheet of the desired thickness andwidth is extruded. In the case of PPE/epoxy resin/amine, this takesplace at a temperature of between 160° C. and 200° C.

[0075] The extruded sheet can then be hardened further, resulting in amarketable intermediate being created. However, for the desiredlaminate, it is advantageous for the extruded sheet further to belaminated with one or two metal skins before the thermoplasticcontaining layer has completely hardened.

[0076] The mean dimension of the epoxy particles which are dispersed inthe PPE network lies in a range from 0.5 to 1 μm when using a mixturecontaining 10% difunctional epoxy resin, and in a range from 1.5 to 3 μmwhen using a mixture containing 20% difunctional epoxy resin. When usinga specific amount of epoxy resin, the number and dimension of the epoxyparticles is dependent on the extent of phase separation achieved. Boththe temperature to which the extruded mixture is heated in order to makethe hardener react with the epoxy resin and the time for which themixture is held at this temperature are decisive factors in a givensystem of PPE/epoxy resin/hardener for the extent of phase separationand therefore the toughness of the core layer.

[0077] The metal skins preferably consist of steel, galvanized steel,stainless steel or an aluminium alloy, or combinations thereof. It isthus possible to create a laminate of which one metal skin consists ofhigh-grade stainless steel while the “reverse side” of themetal-plastic-metal laminate consists of a less expensive steel grade.

[0078] When using an aluminium alloy, an aluminium alloy of a type whichis particularly appropriate for work-hardening is particularly suitablefor the laminate. An alloy which is suitable for hot-hardening is lesssuitable, on account of the process normally used to produce thelaminate.

[0079] In one embodiment of the laminate, the aluminium alloy belongs tothe AA5xxx or the AA6xxx series, such as, without being restricted to,5182, 5054, 6063 and 6016. Aluminium of this type has the right strengthand properties for the intended applications. It is recommended for thelaminate to comprise prestretched aluminium, to prevent so-called Lüderslines from developing during the cold-working of the laminate.

[0080] It has been found that a steel-plastic-steel laminate with layerthicknesses of 0.13/0.65/0.13 mm, respectively, represents an excellentreplacement for 0.70 mm thick steel. At least a PPE/epoxy particlessystem is suitable as a plastic. There is an associated considerablereduction in weight. 0.8 or 0.9 mm steel can be replaced by asteel-plastic-steel laminate, with respective thicknesses of0.15/0.80/0.15, and 1.0 mm steel can be replaced by asteel-plastic-steel laminate with thicknesses of 0.18/0.9/0.18.

[0081] A laminate having a thermoplastic layer that is on one sidesubstantially uncovered, may be somewhat thicker, for instance up to 5mm, or up to 2 mm, because of the strength that must be delivered.

[0082] It will be clear to the person skilled in the art that thesereplacement laminates serve only as an extra indication of how theinvention can be embodied, and that laminates in which the thickness ofone or both metal skins differs from these examples also fall within thescope of protection of the claims.

1. Metal-plastic-metal laminate, comprising a core layer which containsa thermoplastic, which core layer is situated between two substantiallyparallel metal skins and is joined thereto, characterized in that thecore layer furthermore comprises a solvent and a hardener, thethermoplastic being dissolved in the solvent, and the hardener beingcapable of reaction with the solvent.
 2. Laminate according to claim 1,characterized in that the solvent and the hardener each comprise atleast two reactable groups.
 3. Laminate according to claim 1 or 2,characterized in that the reaction product which can be formed from thesolvent and the hardener is not miscible with the plastic.
 4. Laminateaccording to one of the preceding claims, characterized in that thesolvent comprises low molecular weight epoxy resin.
 5. Laminateaccording to claim 4, characterized in that the hardener belongs to thegroup of amines, the group of amides or the group of anhydrides. 6.Metal-plastic-metal laminate, comprising a core layer which contains athermoplastic, which core layer is situated between two substantiallyparallel metal skins and is joined thereto, characterized in that thecore layer comprises polymer particles which are dispersed in thethermoplastic, which polymer particles are formed from a solvent and ahardener which is capable of reaction with the solvent.
 7. Laminateaccording to one of the preceding claims, characterized in that thethermoplastic has only an amorphous phase.
 8. Laminate according to oneof the preceding claims, characterized in that the thermoplastic has aglass transition temperature which lies in a range from 180° C. to 240°C., and preferably in a range from 200° C. to 220° C.
 9. Laminateaccording to one of the preceding claims, characterized in that thethermoplastic belongs to the group of poly(phenylene ethers), includingPPE: poly(2,6-dimethyl-1,4-phenylene ether).
 10. Laminate according toone of the preceding claims, characterized in that the core layer has athickness in a range from 0.5 to 2.5 mm, preferably a thickness in arange from 0.5 to 1.0 mm.
 11. Laminate according to one of the precedingclaims, characterized in that at least one metal skin consists of steel,galvanized steel, stainless steel or an aluminium alloy.
 12. Laminateaccording to claim 11, characterized in that the aluminium alloy belongsto the group of aluminium alloys which are suitable in particular forwork-hardening.
 13. Laminate according to one of the preceding claims,characterized in that the metal skin has a thickness in a range from0.08 to 0.3 mm, preferably has a thickness in a range from 0.12 to 0.19mm.
 14. Laminate according to one of the preceding claims, characterizedin that the laminate comprises an adhesion layer between the core layerand the metal skin.
 15. Laminate according to claim 14, in which thecore layer comprises a thermoplastic which belongs to the group ofpoly(phenylene ethers), characterized in that the adhesion layer belongsto the group of polyurethanes (PUR).
 16. Laminate comprising a plasticlayer which contains a thermoplastic, which plastic layer is joined onone side to a substantially parallel metal skin, characterized in thatthe plastic layer furthermore comprises a solvent and a hardener, thethermoplastic being dissolved in the solvent, and the hardener beingcapable of reaction with the solvent.
 17. Laminate comprising a plasticlayer which contains a thermoplastic, which plastic layer is joined onone side to a substantially parallel metal skin, characterized in that,the plastic layer comprises polymer particles which are dispersed in thethermoplastic, which polymer particles are formed from a solvent and ahardener which is capable of reaction with the solvent.
 18. Laminateaccording to claim 16 or 17, characterized in that the plastic layer isjoined on the other side to a non-metallic skin or body.
 19. Process forproducing a laminate comprising a thermoplastic containing plastic layerthat is joined at least on one side to a substantially parallel metalskin, including a metal-plastic-metal laminate, comprising the steps of:i. providing granules of a thermoplastic, a solvent and a hardener,which hardener is capable of reaction with the solvent; ii. dissolvingthe granules in the solvent, and adding the hardener to form a mixture;iii. extruding the mixture in order to achieve a sheet form; iv. forminga laminate comprising a layer of the extruded mixture and at least onesubstantially parallel metal skin.
 20. Process according to claim 19,comprising the further steps of: v. heating the laminate in order tomake the hardener react with the solvent; and vi. cooling the laminateto a processing temperature.
 21. Process according to claim 20, in whichsteps iv and v are carried out simultaneously.
 22. Process according toclaim 19, in which at least the following step is included after stepiii and before step iv: v. heating the extruded mixture in order to makethe hardener react with the solvent.
 23. Process according to one ofclaims 20 to 22, in which the temperature in step v is higher than theglass transition temperature of the thermoplastic.
 24. Process accordingto one of claims 19 to 23, in which the laminate is formed at atemperature which is higher than the glass transition temperature. 25.Process according to one of claims 19 to 24, in which the plastic isselected from the group of poly(phenylene ethers), and the solvent isselected from the group of epoxy resins, and the hardener is selectedfrom the group consisting of amines, amides and anhydrides.
 26. Processaccording to claim 25, characterized in that the molecular weight of thepoly(phenylene ether) selected is between 20 000 and 30 000 atomic massunits.
 27. Process according to claim 25 or 26, in which between 60 and80% by weight of the mixture comprises poly(phenylene ethers), andpreferably approximately 70% by weight of the mixture comprisespoly(phenylene ethers).
 28. Process according to one of claims 25 to 27,in which the temperature at which the mixture is extruded in step iiilies in a range from 160° C. to 200° C., and preferably in a range from170° C. to 180° C.
 29. Process according to claims 25 to 28, in whichthe temperature at which the granules are dissolved in step ii lies in arange from 170° C. to 200° C.
 30. Process according to one of claims 25to 29, in which the temperature at which the laminate is formed ishigher than 240° C.